Manufacture of superconductors



Oct. 13, 1970 A. c. PRIOR MANUFACTURE 0F SUPERCONDUCTQRS Filed Nov. 29, 1968 I United States Patent ()flice 3,533,919 Patented Oct. 13, 1970 3,533,919 MANUFACTURE OF SUPERCONDUCTORS Arthur Cunningham Prior, Malvern, England, assignor to National Research Development Corporation, London, England Continuation-impart of application Ser. No. 434,924,

Feb. 24, 1965. This application Nov. 29, 1968, Ser.

Int. Cl. C23c 13/02; C2315 17/00 U.S. Cl. 204-38 Claims ABSTRACT OF THE DISCLOSURE A process for depositing a first metal on a second metal which readily acquires a tenacious oxide film includes the steps of evaporating a layer of the first metal on a body of the second metal and electrodepositing a further quantity of the first metal on the resulting body.

The evaporation step may include exposing the second metal to particles of the first metal liberated by electron bombardment in vacuo.

The first metal may be mainly tin and the second metal mainly niobium.

This application is a continuation-in-part of application Ser. No. 434,924, filed Feb. 24, 1965.

The present invention relates to the deposition of one metal on another.

A well-known process for depositing a layer of one metal on another is electrodeposition, by which thick deposits can be built up fairly quickly. However, this process is impracticable for some metals which have a tenacious oxide film, such as niobium. For example, a layer of tin applied to niobium by electroplating will not adhere to the niobium.

It is an object of the invention to provide a process for depositing a first metal on a second metal which has a tenacious oxide film whereby the deposit is more adherent.

According to the present invention a process for depositing a first metal on a second metal which readily acquires a tenacious oxide film includes the steps of evaporating a layer of the first metal on a body of the second metal and electrodepositing a further quantity of the first metal on the resulting body.

The first metal may be mainly tin and the second metal mainly niobium.

An embodiment of the invention will be described by. way of example with reference to the accompanying drawing, which is a diagram, partly in cross-section, of apparatus for tin plating niobium tape by vacuum evaporation of a tin source.

A source 21, made of tungsten or molybdenum, and containing tin is connected via a tube 23 (made of molybdenum or titanium) to a reservoir 25 so that the source 21 may be continually replenished with liquid tin. The source 21, the tube 23 and the reservoir 25 are all heated to keep the tin molten. This is shown schematically by a heater 27. The source 21 is maintained at earth potential.

A circular filament 29 made of 0.015 inch tungsten or tantalum wire is used as an emitter of electrons to bombard the source 21. A total current of some 20 amps AC. is caused to flow in the filament 29. The filament 29 is supported within a hollow electrode 31 having the shape of the solid of revolution of a T about its vertical axis. The electrode 31 is maintained at a negative potential of 4,500 volts, and the filament 29 is connected to it. The whole assembly is held in an evacuated chamber or evacuated chambers. This is indicated schematically by an envelope 33.

In operation at a low rate of evaporation the source 21 is heated principally by bombardment of electrons ejected from the filament 29. In operation at a high rate of evaporation, however, the source 21 is heated by bombardment of electrons ejected by the filament 29, and also by an electric discharge passing through the ionised tin vapour directly between the electrode 31 and the source 21. In action the tin particles evaporated from the source 21 pass against the electron stream through a hole 35 in the electrode 31.

The niobium tape is wound off a reel 37, over four sets of pulleys 3-9, 41, 43 and 45 and (after coating) on a reel 47. To increase the speed of coating the pulleys may be multiple so that the tape passes many times over the evaporation source in such a way that both sides of the tape are coated.

In order to achieve a firmly adherent coating of tin it is necessary for the tape to be heated to some 400-SO0 C. either during or after coating. If the tape is passing slowly over the source it becomes heated sufficiently by radiation and electron bombardment from the filament but at a more rapid passage this heating is not sufficient. The heating can then be conveniently done in the coating apparatus by means of subsidiary electron bombardment heaters 49, 51 which heat the tape after deposition on each pass and before the deposit can be rubbed off by passage over a pulley.

In an alternative arrangement for rapid coating of a large quantity of tape as an alternative to the large number of passes of a single tape a number of separate lengths of tape could be simultaneously passed over the source or over a pair of sources to coat both sides. Each tape would then pass more slowly over the source.

A very thin layer of tin deposited by the process described above is adequate to act as a key to enable a thicker deposit to be built up by electrodeposition, as for example from a potassium stannate bath. This deposit can be built up to the correct thickness for producing the desired quantities of niobium tin.

After electroplating it may be advantageous to heat the tape more rapidly to 860 C. or above in order to improve the quality of the deposit and its adhesion to the niobium.

Obviously the process would work in the same way if the source 21 contained an alloy which was mainly tin. Similarly the tape could be an alloy which was mainly niobium. Furthermore the process may be applicable to the plating of other metals which readily acquire tenacious oxide films, for example, aluminum.

I claim:

1. A method of depositing a first metal composed mainly of tin on a second metal composed mainly of niobium and which readily acquires a tenacious oxide film, which process comprises the steps of: bombarding said first metal in liquid form with electrons from an electron emitter in vacuo to evaporate said first metal; contacting a body of said second metal with vapours of the evaporated first metal to form a vapour-deposited layer of said first metal on said body; a large voltage difference being maintained between said electron emitter and both said first metal and said body; and electrodepositing a further quantity of said first metal on the body over the vapour-deposited layer to provide an adherent electrodeposited layer of said first metal on said body.

2. A method according to claim 1 wherein said body is unheated prior to contacting the vapours of the evaporated metal and is heated to a temperature of from 40 0500' C. after contacting said vapours.

3. A method according to claim 1 including the further step of heating the body provided with the electrodeposited layer at a temperature of at least 860 C.

4 4. A method according to claim 1 wherein said first 3,021,271 2/ 1962 Wehner 204192 inetal consists essentially of tin and said second metal 2,993,266 7/1961 Berry 2'9'25.42 consists essentially of niobium. 3,443,304 5/1969 Maier 29472.1 5. A method according to claim 4 wherein said first 3,346,467 10/1967 Allen 204-37 metal and said'body are electrically grounded.

- 5 HOWARD s. WILLIAMS, Primary Examiner References Cited R. L. ANDREWS, Assistant Examiner UNITED STATES PATENTS 3,305,473 2/1967 Moseson 204 29s 3,253,331 5/1966 'Limansky 29473.1 10 117 107;204 192 3,178,308 4/1965 Oxley et a1. 117-106 

